Naturally acquired infections with Francisella tularensis, the bacterial agent of tularemia, occur infrequently in humans. However, the extraordinary infectivity of the organism, its high rate of lethality in man, and its ability to be disseminated by aerosols raise serious concerns that it might be exploited as an agent of bioterrorism. F. tularensis evades the antimicrobial weaponry of macrophages and instead proliferates within these cells. Moreover, our preliminary data indicate that live F. tularensis stimulates only selected proinflammatory functions of vascular endothelium. We therefore hypothesize that F. tularensis interacts with the innate immune system in an exceptional manner that contributes to the high infectivity and virulence of the organism. To test this premise, both attenuated and fully virulent strains of F. tularensis will be used to pursue the following specific aims: 1) To examine the survival and proliferation of F. tularensis within phagocytes and endothelium. Bacterial components that permit intracellular growth will be identified through genetic approaches, and DNA microarrays will be used to compare gene expression in bacteria growing intracellularly or extracellularly. 2) To elucidate the proinflammatory responses of endothelial cells and leukocytes to F. tularensis. Both specific and global responses of the host cells will be examined, the latter through microarray analysis. The ability of infected leukocytes to traverse endothelium, a possible mode of dissemination of the organism, also will be assessed. 3) To identify bacterial components and host receptors that mediate proinflammatory interactions between F. tularensis and cells of innate immunity, using both genetic and biochemical strategies. Bacterial and host factors that are implicated in the pathogenesis of tularemia through in vitro studies will be further examined in normal or genetically altered mice inoculated with aerosols of wild-type or mutant strains of F. tularensis. Collectively, the results of these studies will shed much-needed light on the apparently novel ways in which F. tularensis interacts with cells of the innate immune system. It is further anticipated that this knowledge will help in the design of rational strategies to minimize the human toll should F. tularensis be unleashed in an act of bioterrorism.